During embryogenesis, mesoderm formation and specification represent the first steps in the development of multiple organ systems such as blood, endothelium, heart and skeletal muscle. Much of our understanding of the factors that govern stages of commitment has come from Xenopus, Zebrafish and chick embryo, models that provide access to the early embryo. The mouse embryo is less amenable to such experimental manipulations. The in vitro differentiation of mouse ES cells offers an approach for investigating early stages of lineage commitment. Using an ES cell line in which the onset of mesoderm can be detected, studies have demonstrated that it is possible to track and isolate mesoderm populations. This system recapitulates the temporal pattern of mesoderm induction and specification in the embryo. The overall goals of this proposal are to define the mechanisms that regulate the specification of mesoderm to the hemangioblast and cardiac fates using this model system. The specific aims of the approach are as follows: 1) Characterization of hemangioblast and cardiac mesoderm. 2) Define the molecular regulators of cardiac and hemangioblast mesoderm.